I have the Kamikaze 4-1 header....had a 2.5" downpipe made to go with the Contour pan (all the way to the cat)then 2.25" piping back to the muffler.....the o2 sensor was relocated to the other side due to the a/c being in the way. As for power.....I gained 10Hp and torque stayed the same....but I gained 19hp from 5600-6500 rpms and 13 ft-lbs.
